The piezoelectric type liquid level sensor is constituted for a bimorph type liquid sensing unit by liquid-tightly bonding a vibration plate to the lower end opening of a metal protection conduit so that one surface of the vibration plate is externally exposed and by bonding a piezoelectric element to the inner surface of the vibration plate. In such a level sensor, the fibration plate is disposed at a height corresponding to a predetermined liquid level, and when the liquid in a vessel is at a predetermined level or higher, the outer surface of the vibration plate is dipped in the liquid. Accordingly, even if an alternating voltage is applied to the upper and lower surface electrodes of the piezoelectric element, the element cannot vibrate because of the hydraulic pressure of the liquid. If the liquid level is at the predetermined level or lower, the outer surface of the vibration plate is exposed to the atmosphere, the application of the hydraulic pressure of the liquid is released, and the element can be vibrated. Thus, the fact that the level of the liquid is at the predetermined level or lower can be detected by detecting the vibration of the element.
Such an arrangement is heretofore known. One example of conventional arrangements is shown in FIG. 1 of the accompanying drawings, in which a vibration plate A is provided with a piezoelectric element B having an exciting moving electrode C and a feedback electrode D on one surface and a common electrode E on the other surface, and is oiltightly bonded to the lower end of a metal body conduit F having a suitable length, a cable G is mounted on a packing H provided into an upper end port of the conduit F, and lead wires I, J and K extending from cable G are respectively connected to the electrodes C, D and the conduit F.
In the aforementioned known piezoelectric type liquid level sensor, the lead wires I, J and K must be respectively passed through the interior of the conduit F, and bonded to the vibration plate A and the body conduit F, and the packing H must be engaged with the upper end port of the conduit F so as to assembly it. Further, in order to absorb the vibration and fluctuation applied to the conduit F, the lead wires must be slackened in the conduit F. However, since the lead wires extending from the vicinity of the packing are long and heavy in weight, almost all of the weight of the lead wires is applied to the piezoelectric element B. This disturbs the vibration of the vibration element including the vibration plate A and the piezoelectric element B when voltage is applied, and thus a noise is induced in a detection signal on the lead wire J.
The frequency of the alternating voltage applied to the piezoelectric element is set so that it coincides with the resonance frequency of the liquid sensing unit in which the vibration plate is bonded to the piezoelectric element so as to improve the vibration of the piezoelectric element upon application of the voltage to the element. The atmospheric temperature of the position for mounting the level sensor might increase such as where the sensor is used to detect the liquid level of oil in an oil tank of an automobile. In this case, the resonance frequency of the sensing unit is increased. Thus, if the liquid level lowers so that the surface of the vibration plate is exposed to the atmosphere, the piezoelectric element may not vibrate corresponding to the applied voltage, the output signal from the sensor may drop, or a noise may be produced, thereby deteriorating the performance of the sensor.
It is, therefore, an object of this invention to provide a method of fabricating a piezoelectric type liquid level sensor in which the influence of the weight of the lead wires on the vibration element is eliminated and in which assembly of the sensor is facilitated.
Another object of the invention is to provide a piezoelectric type liquid level sensor for detecting whether the level of various liquids is a predetermined value or higher or not, in which the influence of the weight of the lead wires on the vibration element is eliminated.
A further object of the invention is to provide a piezoelectric type liquid level sensor in which the frequency band width capable of producing a preferable output signal is increased.